Method and a Device for Power Reduction in an LTE System

ABSTRACT

A method for use in a wireless communications system in which there is at least a first mode which controls the traffic to and from user terminals in a cell within the system, so that there is downlink traffic in the system. The first node transmits downlink traffic in radio frames, each of which comprises sub-frames. The first node performs measurements on pre-defined system indicators in at least said first cell, and based on the results of said measurements, the first node is allowed to autonomously decide to vary the number of available down link sub-frames used for down link traffic in said down link radio frames and also to vary the content of the down link sub frames which are used, said decision being valid for a time which is specified by the first node.

TECHNICAL FIELD

The present invention discloses a method and a device for powerreduction in a wireless communications system.

BACKGROUND

In a wireless communications system such as, for example, a system ofthe LTE (Long Term Evolution) kind, there will be one or morecontrolling nodes, so called base stations, sometimes referred to aseNodeB, depending on the specific kind of system. One role for a basestation is to control all traffic to and from user terminals within acertain geographic area in the system, a so called cell.

A base station in a cellular system will comprise one or moretransmitters, each of which in turn comprises one or more poweramplifiers, PAs. The PAs are one of the main consumers of energy in abase station, since the PAs are used to amplify input signals with lowinput power to output signals with high output power, which is due tothe fact that a high output power level is required to provide adequatecoverage and high data rates in a cellular network.

A PA in an average base station has an output power of approximately 20W and an efficiency level of around 20%, which means that approximately100 W are needed in order to obtain a PA with an output power of 20 W.Reducing these power levels would thus mean major savings in energy, andwould also lead to further savings in energy due to, inter alia, reducedcooling needs.

In many cellular systems, both those comprising an FDD (FrequencyDivision Duplex) mode and those comprising a TDD (Time Division Duplex)mode, the base stations transmit in so called radio frames, each ofwhich will comprise a number of sub-frames.

As can be understood from the explanation given above, a reduction inthe energy consumed by a PA in a wireless communications system would behighly beneficial, both in order to reduce operator expenditure (OPEX)and for environmental reasons. One way of achieving this would be toreduce the number of sub frames in which transmission is made from thebase station, or to reduce the transmission in certain sub frames. orentirely “shut down”.

SUMMARY

Thus, as has emerged from the above, there is a need for a solution bymeans of which the energy consumed by a PA in a communications systemcould be reduced, particularly by means of identifying sub frames inwhich the transmissions could, at least temporarily, be reduced orperhaps entirely shut down. The solutions should also make it possibleto utilize such sub frames for “full transmission”, if and when such aneed arises.

Such a solution is offered by the present invention in that it disclosesa method for use in a wireless communications system in which there isat least a first node which controls the traffic to and from userterminals in a certain first geographical area, a cell, within thesystem.

In a system in which the invention is applied, there will thus bedownlink traffic in the system, and the first node will transmit itsdownlink traffic in radio frames, each of which comprises a certainnumber of sub-frames.

According to the inventive method, measurements are performed onpre-defined system indicators in at least the first cell. Based on theresults of said measurements, a decision is made to vary the number ofavailable down link sub-frames which are used by the first node for thetransmission of down link traffic in said down link radio frames, adecision which is valid for a certain amount of time.

Suitably but not necessarily, the decision also comprises varying thecontent of the down link sub-frames which are used.

In one embodiment of the invention, the decision is made autonomously bythe first node, including the length of the decision's validity, and inanother embodiment, the results of the measurements are communicated toa central node in the system, with the central node taking the varyingdecision, including the length of the decision's validity, andcommunicates it to the first node for implementation.

Thus, in one embodiment of the inventive method, a base station of acellular or other system may save energy in its PAs by varying thenumber of used downlink sub-frames by declaring some of them “idle”,i.e. that no transmission will take place in those sub-frames, or acentral node makes a corresponding decision for one or more basestations. Also, in one embodiment, the first node or the central nodemay decide to vary the number of used downlink sub-frames by means ofdeclaring some of them “active”, i.e. that transmission will take placein previously idle sub-frames.

The pre-defined system indicators which are measured, suitably by thefirst node, in order to decide how to use the available sub frames inthe down link can include the system load in the first cell, so that, ifthe system load is below a certain threshold, a certain amount of subframes can be declared “idle”, or, conversely, if the system load risesabove a certain threshold, previously idle sub frames may be activated,i.e. used for transmission. The pre-defined system indicators may alsoinclude the system load in at least one other cell in the system, aswell as the interference level in the first cell.

Another possible such system indicator may include the interference inthe first cell.

The invention also discloses a transceiver which could be used as aneNodeB in a system of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail in the following, withreference to the appended drawings, in which

FIG. 1 shows an overview of a system in which the invention can beapplied, and

FIG. 2 shows a HARQ protocol, and

FIGS. 3-5 show HARQ signalling of one aspect of the invention, and

FIG. 6 shows a rough flow chart of a method of the invention, and

FIG. 7 shows a rough block diagram of a transceiver of the invention.

DETAILED DESCRIPTION

FIG. 1 shows a schematic overview of a part of a system 100 in which theinvention may be applied. As has been explained above, the invention isprimarily intended for a wireless cellular LTE system which has beenconfigured to use either the TDD or FDD mode, so references below to oneof those principles should merely be seen as examples intended tofacilitate the reader's understanding of the invention.

In addition, the invention may also be applied to other kinds ofwireless transmission systems, which are not cellular.

Returning now to FIG. 1, the system 100 shown in FIG. 1 comprises atleast one base station 110, usually referred to as eNodeB in LTE. Thisbase station may also be seen as a first node in the system 100, and hasas one of its tasks to control traffic to and from user terminals in acertain geographical area 120 in the system, such an area being referredto as a cell. In FIG. 1, two user terminals, UEs, are symbolically shownas 130 and 140. The number of UEs within a cell is of course an exampleonly.

As shown in FIG. 1, the system 100 may also comprises a central node150, which has as one of its tasks to control the function of one ormore of the eNodeBs in the system 100. An example of such a node may bea so called RRM-Node, Radio resource Management Node, which has amongits tasks the carrying out of RRM algorithms which demand multi-cellknowledge in order to improve the system performance.

In the system 100 for which the invention is intended, the transmissionsfrom the base station 110 to the UEs, the so called downlink, DL,direction for traffic in the cell 120, is divided into so called radioframes, each of which comprises a certain number of so called subframes.

As an example, the radio frame of Frame Structure 1 in E-UTRAN consistsof 10 sub-frames, each of 1 ms duration, and each sub-frame consists of2 slots of 0.5 ms duration and 7 OFDM symbols. 12 sub-carriers of 15 kHzin the frequency domain and 0.5 ms duration are defined as one ResourceBlock.

As is known to those skilled in the field, the following can be saidabout a Radio Frame:

-   -   Reference Symbols are placed in each resource block of a radio        frame.    -   The PDCCH (Physical DL Control Channel, used for signalling both        downlink scheduling assignments and uplink scheduling grants) is        spread over the whole carrier frequency and uses a variable        amount of resources (1-3 OFDM symbols) in each sub-frame.    -   The PBCH (Physical Broadcast Channel) is placed in the middle of        the carrier (the centre 1.25 MHz) and appears in sub-frames 0        and 5 of each radio frame. This channel contains system        information which is broadcast in the cell.    -   Another part of the system information is mapped onto the PDSCH.        In this description, such information is denoted as the dynamic        BCH or D-BCH.    -   The Primary Synch Channel, P-SCH1, appears on one OFDM symbol in        the centre 1.25 MHz in sub-frames 0 and 5 of each radio frame.    -   The Secondary Synch Channel #1, S-SCH1, appears on one OFDM        symbol in the centre 1.25 MHz in sub-frames 0 of each radio        frame.    -   The Secondary Synch Channel #2, S-SCH2, appears on one OFDM        symbol on the centre 1.25 MHz in sub-frames 5 of each radio        frame.

In addition, the first few OFDM symbols that are used for PDCCH willalso contain the PHICH, Physical HARQ Indicator Channel, which is usedto convey acknowledgements or negative acknowledgements for the uplinkHARQ protocol, with data being sent in the uplink and reception statusis sent in the downlink.

Resources which are not explicitly listed in the bullet list above areunderstood to comprise resources which are used for the PDSCH (PacketDownlink Shared Channel), a channel which is used to send user data tothe UEs served by the eNodeB. It should be pointed out that certain OFDMsymbols contain only PDSCH.

It should be noted that the exact structure of the radio frames and themapping of control and data channels onto the radio frames is an ongoingwork in 3GPP. Hence, the description provided in this text is usedmerely as an example in order to further the reader's understanding ofthe invention, and it should be understood that variations in the radioframe structure may occur, with the invention being equally applicableto systems with radio frames with a structure other than the exact onedescribed here.

Modern cellular packet-switched communication systems such as LTEsystems, for which the invention is mainly intended, as well as HSPA(High Speed Packet Access) systems, which are both specified in 3GPP,employ a Hybrid ARQ (Automatic Repeat ReQuest) protocol in theirrespective MAC (Medium Access Control) layer. The basic functionality ofthe HARQ protocol is to correct block errors that occur over the airinterface.

The HARQ protocols specified in LTE and HSPA utilize so-called HARQprocesses to transfer the data. The HARQ processes are used to associatea potential retransmission to its original transmission in order toenable soft combining at the HARQ receiver. Only when the HARQ receiverhas reported correct reception of the data sent on a HARQ process may itbe used to transmit new data. Consequently, before the reception of aHARQ status report from the receiver, the HARQ sender cannot know if itshould send new data or a retransmission of the “old data”. In themeantime, it therefore, “stops and waits” until it knows the result ofthe transmission. In order to still be able to utilize the down linkduring these waiting periods, it is customary to use multiple parallelsuch HARQ processes.

Further, two main HARQ protocol modes exist:

1. Synchronous HARQ, in which potential retransmissions occur at apre-determined time after the initial transmission. In this case, noHARQ process number needs to be transmitted, since the process number isimplicitly identified by the time of its transmission. This type ofoperation has been chosen for the LTE uplink HARQ protocol.

2. Asynchronous HARQ, in which there is no strict timing relationshipbetween a transmission and its retransmission. Instead, the HARQ processnumbers are explicitly signalled in each information block. This type ofoperation has been chosen for the LTE downlink HARQ protocol.

Reference will now be made to FIG. 2 in order to exemplify thesynchronous HARQ mode. In FIG. 2, the uplink HARQ sender, the UE, uses 7parallel HARQ processes in a stop and wait fashion. The numbers belowthe UE uplink sub-frames in FIG. 2 show which HARQ process that is usedfor transmission in a particular sub-frame. Data from the different HARQprocesses are sent in a “round-robin” fashion, making explicitsignalling of process numbers unnecessary.

It should be pointed out that when using a HARQ protocol, transmissionof data in one direction, in FIG. 2, uplink transmission, requiresstatus signalling in the opposite direction, i.e. in the downlinkdirection in FIG. 2, as is indicated in the figure.

As has been pointed out initially in this text, in present day wirelesscellular communications systems, it is a strong desire to reduce thepower expenditure in the PAs in the base stations, which could suitablybe done by limiting the number of downlink sub frames which are used bythe PAs, or by limiting the extent to which the DL sub frames are usedby the PAs.

However, in present solutions, it is difficult to find downlinksub-frames which could be “switched off”, or declared idle, due to thewide variety of protocols/channels which need to be supported on thedownlink frame structure.

A purpose of the present invention is to provide solutions which make itpossible to “free” certain sub frames from the need of communication, orat least to reduce the degree to which they are utilized

In order to provide a solution to the problem of finding sub frameswhich may be declared idle, or have their content reduced, the presentinvention proposes a method by means of which a base station mayautonomously, or as instructed by a central node in the system, declarecertain sub-frames in one or more cells which it serves as “idle”. Inthese idle sub-frames, no energy is transmitted from the base station,or a reduced amount of information is sent in them, by means of whichthe energy consumption is reduced.

According to the invention, in order to find sub frames which may bedeclared idle or have their information content reduced, measurementsare performed, suitably by the base station, on certain pre-definedsystem indicators in at least a first cell, and, based on the results ofthese measurements, the base station is allowed to autonomously, decideto vary the number of available down link sub-frames which are used forthe transmission of down link traffic in said down link radio frames andsuitably also to vary the content of the down link sub frames which areused. The decision taken by the base station is allowed to be valid fora certain amount of time, which is also specified by the base stationitself.

As an alternative, the decision to vary, including the validity of thedecision, is taken by a central node in the system, and thencommunicated to the first node, the eNodeB, for implementation. In suchan embodiment, the measurements are suitably carried out by the firstnode/eNodeB, and the results of the measurements are then communicatedto the central node, which uses the results of the measurements to baseits decision on. One example of a central node for such use is a socalled RRM-node, as described previously in this text.

The measuring mechanism or mechanisms will be elaborated on in moredetail later in this text, but briefly, the decision to vary the numberof used downlink sub-frames may be done by declaring some of the subframes in question “idle”, i.e. no transmission will take place in thosesub-frames, or, conversely, the decision may also be that transmissionwill take place in previously idle sub-frames, if the system indicatorswhich are measured indicate that this is suitable, or that there is aneed for this.

As another alternative, instead of declaring one or more sub-framesidle, the amount of information sent in the sub-frame or frames mayinstead be reduced, in order to reduce the energy expenditure in thosesub-frames. If such a “reduction mechanism” is introduced in a system ofthe invention, this could be done in the following manner: theinformation which is necessary in order to maintain the frame structureof a sub-frame is transmitted, including, for example, PDCCH, PHICH, andRS, but no data is actively scheduled for transmission, including anydata which is scheduled on PDSCH, inclusive of page and D-BCH as well asdata to UE:s which are waking up from DRX, and “regular data” which isassociated with a scheduling assignment on PDCCH.

Turning now to the matter of the pre-defined system indicators which aremeasured, suitably by a function in the eNodeB in order to decide tovary the amount of used and contents of downlink sub-frames, one suchindicator is suitably the system load in a cell of the eNodeB. (Theexpression “a cell” is used here, since one eNodeB may control one ormore cells in a system.) One way to measure the system load for thispurpose is to measure what percentage of the resources available totransmit user-data was actually used to transmit user data. Thispercentage can preferably be measured to reflect a time-average over acertain time interval. This indicator would then vary from 0 to 100%.

Thus, as an example, if the system load is below a certain threshold,the eNodeB on its own, or as instructed by a central node, may decide todeclare a certain number of down-link sub frames as idle, and if thesystem load is below another lower threshold, an additional number ofdown-link sub-frames may be declared idle. Conversely, if a number ofsub frames are idle, and the system load rises above one or morethresholds, the number of idle down link sub frames may be reduced,suitably in steps, as the system load increases.

As an alternative or a complement to the system indicators which havebeen mentioned above, the pre-defined system indicators may also includethe system load in at least one other cell in the system. This may then,as an example, be used in the following way: if one or more neighbouringcell or cells have a high load, sub-frames in the own cell are notdeclared as idle, so that UEs from the cells with a high load nay behanded over to the own cell.

An example of yet another possible pre-defined system indicator is theinterference in the cell: if there is a high degree of interference incertain down-link sub-frames, those sub-frames may be declared idle, andthe traffic may be diverted to other sub frames.

If the decision is made not to utilize certain sub-frames for downlinktransmission, the eNodeB will, according to the invention, abstain fromtransmitting any of the following in those sub-frames:

-   -   The RSs (Reference Symbols) in the idle sub-frames will not be        transmitted.    -   No control signalling mapped to the channels SCH (any of the        primary or secondary), PCH or PBCH will be sent.    -   No other system information (D-BCH) will be sent.    -   No scheduling assignment for the downlink (with associated data)        will be sent in the downlink sub-frames.    -   No scheduling grant for the uplink should be sent in the        downlink sub-frames.

In addition, suitably, no data should have been scheduled in an uplinkframe such that HARQ feedback is expected in a DL idle sub-frame, andpreferably, no UE should have been configured to “wake up” from DRXduring an idle DL sub-frame.

Furthermore, in yet another embodiment of the invention, no UE shouldhave been configured to listen to a page on the PCH during such an idleDL sub-frame.

According to the invention, UEs that are served by the eNodeB inquestion are informed about which sub-frames that are idle in the cell,which may be done in the following manner:

For UEs that are active, i.e., UEs that are RRC_Connected in the cell atthe time when the decision is made to declare one or more sub-framesidle (or declared active from an idle state) this can be achieved bymeans of RRC signalling, or via a broadcast message or SI signalling.

UEs that enter the cell as a result of a so called handover need to beinformed about which sub-frames are idle/active before entering thecell. This can be done via the handover signalling (e.g., the Handovercommand message).

UEs which power on in the cell in question can receive this information(idle/active sub-frames) from the System Information.

The mechanisms used in order to make certain sub-frames idle, asdescribed briefly above, will now be described in more detail:

No Transmission of Reference Symbols, RSs:

In order for the PA of a eNodeB to be idle during a sub-frame, the PAshould not need to amplify any signal containing RSs. Thus, the eNodeB,and consequently its PA does not transmit any RSs during idlesub-frames.

No Transmission of Control Signalling such as SCH, PCH. P-BCH or D-BCH.

No control signalling such as SCH, PCH or P-BCH or D-BCH should betransmitted in idle sub-frames. The placement of control signalling suchas SCH, PCH or P-BCH is typically standardized, so that its placement inthe radio frame is static. The reason for this is that UEs entering thecell, or operating in some form of sleep mode, should know where andwhen to find the necessary information.

For example, SCH and P-BCH information is placed in sub-frames 0 and 5according to the LTE standard, and for that reason, those sub-framesshould, if possible, not be declared idle. Naturally, the numbers (0 and5 in this example) of the sub-frames used for this control signallingmay vary, in which case the sub-frames which should not be declared asidle will also vary.

No Scheduling Assignment for the Downlink (with Associated Data) shouldbe Sent in the Downlink

No scheduling assignment for the downlink, with associated data, shouldbe sent in the downlink, which is controlled by the downlink schedulingfunction in the eNodeB. During idle frames, the downlink scheduler doesnot schedule transmission of any data.

No Scheduling Grant for the Uplink Should be Sent in the Downlink

No scheduling grant for the uplink should be sent in the downlink, whichshould be controlled by the uplink scheduling function in the eNodeB.The uplink scheduler should not schedule any grants during sub-framesthat have been declared idle.

No HARQ Feedback in Downlink

No data should have been scheduled in an uplink frame such that HARQfeedback is expected (by the UE) in a DL idle sub-frame, as controlledby the uplink scheduler function in the eNodeB.

However, one issue that needs to be considered here is that in a typicalcase, a HARQ feedback which arises from an original transmission and anypotential HARQ retransmissions are not sent in the same sub-frame. Thisprinciple is shown in FIG. 3 for the case where the eNodeB has beenconfigured to use 7 HARQ processes.

In the example shown in FIG. 3, the eNodeB is assumed to need 3 TTIs inorder to calculate the feedback. The figure shows the originaltransmission (“T1”) and first two retransmissions (“R1” and “R2”) thatare sent on the uplink on HARQ process 1, and the associated HARQfeedback on the downlink is indicated as NACK T1, NACK R1 and NACK R2.

As is visible from the figure, the HARQ feedback is not sent in the samesub-frame number for the original transmissions and the subsequentretransmissions. Given this, it becomes increasingly difficult tointroduce any idle sub-frames.

Although the exact processing times required in the eNodeB and thenumber of HARQ processes may differ compared to the assumptions made inthis example, the principle of the problem highlighted in the examplewill exist in many of the possible configurations. One way of solvingthe problem described above is to:

-   -   1. Reconfigure the number of HARQ processes used in the uplink        HARQ so that it is equal to the number of TTIs per sub-frame in        which the eNodeB is allowed to give UL (UpLink) grants. This may        involve reconfiguring a UE that moves between cells that have a        different number of idle sub-frames per radio frame. In certain        cases, this may also involve purging the HARQ buffers on the UE        side if there is still data in the processes when the number of        HARQ processes is decreased.    -   2. Provide the UE and the eNodeB with rules which provide a        unique mapping between HARQ process and the HARQ feedback in the        downlink. This rule or rules is/are preferably standardized.

One example of the principle described in items 1 and 2 above is shownin FIG. 4. In the figure, downlink sub-frames 2, 3, 4, 6, 7, 8 and 9have been declared idle by the eNodeB. Uplink scheduling grants can besent in sub-frames 0, 1, and 5, and consequently, the UE has beenconfigured with 3 uplink HARQ processes, as described above under theheading “No scheduling grant for the uplink should be sent in thedownlink.”

The uplink grant for HARQ process 1 is sent in downlink sub-frame number0. During “normal” operation, i.e. without downlink idle frames, thecorresponding HARQ feedback would have been expected after a delay ofT_(HFB), i.e., in downlink sub-frame 7 in FIG. 4. However, since thatsub-frame has been declared as idle, the UE needs to find anothersub-frame where the HARQ feedback is expected.

One possible way for the UE to calculate where the feedback shouldappear is to let the HARQ feedback appear in the next non-idle sub-framefollowing a delay of T_(HFB) after transmission of the data, where noother HARQ process is waiting for HARQ feedback. This rule would alsoneed to be shared by the eNodeB, so that the feedback is sent andreceived at the right time.

The part of the paragraph above which includes the feature that no otherHARQ process is waiting aims at resolving the situation of HARQ processnumber 2 in the figure. The first non-idle sub-frame following a delayof more than T_(HFB) film would have been sub-frame 0. However, the HARQfeedback for HARQ process 1 is expected there, which is why HARQ process2 needs to receive its feedback in sub-frame 1.

Thus, in a system which utilizes the present invention, it is possibleand suitable to decrease the number of HARQ processes used for theuplink in relation to the number of sub-frames where the eNodeB isallowed to give uplink grants, and to also provide a unique mappingbetween HARQ process and the HARQ feedback in the downlink, as shown inthe example of FIG. 4 and described above.

No “Wake-Up” from DRX

No UE should have been configured to “wake up” from DRX during an idleDL sub-frame.

A UE in DRX only “wakes up”, i.e., switches on its transmitter andreceiver, at predetermined time instances in order to save power. Thesetime instances are known both at the UE and the eNodeB, and areconfigured by the eNodeB. At those time instances, the eNodeB mayschedule data in the downlink to the UE. Hence, if a UE's wake-up periodcoincides with idle DL sub-frames, that UE would not be able to becontacted from the eNodeB. However, since the eNodeB determines whichsub frames that are to be idle, as well as configuring the “wake-up”times of the UEs, the eNodeB thus has all of the information necessaryin order to configure the wake-up times so that they do not coincidewith idle sub-frames.

In fact, it would also be possible to let the eNodeB comprise amechanism to configure all UEs (in the cell or cells in question) DRXcycles so that their wake-up periods do not coincide with idle DLsub-frames. If such a solution is for some reason not feasible, e.g.,the configuration mechanisms that are available under the standard usedare not flexible enough, it would be possible to include a rule in thesystem for both the UES and the eNodeB which would state that “if theconfigured wake-up time instance coincides with an idle DL sub-frame,the next non-idle sub-frame following that idle sub-frame should countas the wake-up period”.

No Paging Expected

No UE should have been configured to listen for a page during an idlesub-frame. Suitably, this is handled in the same manner as with theDRX—wake up situation, as described above.

An additional feature which is envisioned within the scope of thepresent invention is as follows:

As explained above, one purpose of the present invention is to reducepower consumption in the eNodeB by switching off the PA in certain“idle” DL sub-frames. In the UL, on the other hand, it is in principlestill possible to utilize all sub-frames. Furthermore, for a UE that ispower limited in the UL, i.e. is transmitting at maximum power, it isdesirable to be able to transmit in as many UL sub-frames as possible.

However, as explained above, it will only be possible to send UL grantsand HARQ feedback during non-idle DL sub-frames. To still be able toallow a UE to transmit in any UL sub-frame, one solution would be toallow an UL scheduling grant to be valid for more than one UL sub-frame.This is illustrated in. FIG. 5, where Grant P1 is valid for one ULsub-frame, Grant P2 is valid for 4 UL sub-frames and Grant P3 is validfor 5 sub-frames. Thus, the core of this feature is to allow UL grantsfor multiple TTIs.

The number of mechanisms described above for making sub-frames idle mayalso be used to reduce the transmissions in a DL sub-frame, if it isdesired to maintain the sub-frame as active, but with a reduced contentin order to obtain some degree of energy saving. In other words, anotherway of offered by the present invention reducing power consumption in aneNodeB is by switching off the PA in the eNodeB for a subset of theduration of an “active” DL sub-frame.

In one embodiment, the DL sub-frame content is varied such that the datawhich is to be transmitted an the PDSCH in a radio frame is concentratedto a subset of the subframes in said radio frame. Preferably, but notnecessarily, this concentration is implemented in a scheduling functioncontained in the eNodeB. This concentration frees some subframes fromPDSCH transmission. As previously noted, some OFDM symbols of a subframecontain only PDSCH transmission. Consequently, during such OFDM symbolsin a subframe in which no PDSCH transmission occurs, the PA can beswitched off during such an OFDM symbol, thereby giving energy savings.

FIG. 6 shows a rough flow chart of a method 600 of the invention. Stepswhich are options or alternatives have been indicated with dashed lines.

As shown in step 610, according to the invention, measurements areperformed on pre-defined system indicators in at least a first cell inthe system, and, as shown in step 615, based on the results of thesemeasurements, a decision is made to vary the number of available downlink sub-frames which are used by a first node such as an eNodeB for thetransmission of down link traffic in said down link radio frames, adecision which, as shown in step 625, is valid for a certain amount oftime.

As shown in step 640, the decision may also comprise varying the contentof the down link sub-frames which are used.

Step 645 illustrates that the “varying” decision may be madeautonomously by said first node, including the length of the validity ofthe decision, and step 650 shows that, alternatively, the results of themeasurements are communicated to a central node (“RRM node”) in thesystem, with the central node taking the varying decision, including itsvalidity, and communicates the decision to the first node forimplementation,

Step 630 shows that the “varying decision” may comprise varying thenumber of used downlink sub-frames by declaring some of them “idle”,i.e. that no transmission will take place in those sub-frames, and step635 shows that the decision may also be to vary the number of useddownlink sub-frames by means of declaring some of them “active”, i.e. sothat transmission will take place in previously idle sub-frames.

FIG. 7 shows a rough block diagram of a “base station” or a transceiver700 of the invention, for use as a eNodeB as described above. As can beseen in FIG. 7, the eNodeB 700 of the invention comprises an antenna 710for communicating with the UEs in one or more cell, and also comprises atransmitter 730 and a receiver 720. In addition, the eNodeB 700 alsocomprises control means such as for example a microprocessor 740, aswell as comprising a memory 750. In addition, the transceiver 700 maycomprise an interface, “Int”, 760, towards a central node, such as anRRM node, in the system.

The transceiver 700 basically comprises means for functioning accordingto the method described above, and thus comprises means for controllingthe traffic to and from user terminals in a cell. Suitably, those meanscomprise the antenna 710, the receiver 720, the transmitter 730, thecontrol means 740 and the memory 750.

In addition, the transceiver 700 also comprising means for transmittingdownlink traffic in radio frames, each of which comprises a certainnumber of sub-frames, said means suitably comprising the antenna 710 andthe transmitter 730.

In addition, the transceiver comprises the control means 740 and thememory 750 which enable it to perform measurements on pre-defined systemindicators in at least said first cell, and also comprises means suchas, for example, the antenna 710, the transmitter 730, the control means740 and the memory 750 for varying the number of available downlinksub-frames which are used for the transmission of down link traffic insaid downlink radio frames for a certain period of time.

The transceiver 700 may additionally use the controller 740 and thememory 750 for varying the content of the downlink sub frames which areused.

In one embodiment, the transceiver 700 additionally comprises means 760,i.e. the interface towards the central node such as an RRM node in thesystem, which allow the transmitter to transmit the results of themeasurements to such a central node in the system, and to receiveinstructions from the central node regarding the varying, as well as thevalidity in time of said varying.

In another embodiment, the controller 740 and the memory 750 may alsoenable the transceiver to autonomously make the decision regarding saidthe, based on the measurements mentioned above.

The antenna 710, the transmitter 730 and the control means 740 may alsobe used by the transceiver 700 in order to vary the number of useddownlink sub-frames by declaring some of them “idle”, i.e. so that notransmission will take place in those sub-frames, and/or for varying thenumber of used downlink sub-frames by means of declaring some of them“active”, i.e. that transmission will take place in previously idlesub-frames.

The measuring means, primarily the control means 740 and the memory 750may also include among the pre-defined system indicators the system loadin the cell, the system load in at least one other cell in the system,and/or the interference in the cell.

As has also emerged from the description above, the transceiver 700will, if certain sub-frames aren't used for down link transmission,abstain from transmitting any of the following in said certainsub-frames:

-   -   The RSs should not be transmitted in the idle sub-frames.    -   No control signalling mapped to the channels SCH (any of the        primary or secondary), PCH or PBCH will be sent.    -   No scheduling assignment for the downlink, with associated data,        should be sent in the downlink.    -   No scheduling grant for the uplink should be sent in the        downlink.

In addition, if some of the available down link sub-frames are notutilized for transmission, the transceiver does not schedule data in anuplink frame so that HARQ feedback will be expected in a DL idlesub-frame, and suitably, does not configure any UE to “wake up” from DRXduring an idle DL sub-frame.

Also, if some available down link sub-frames will not be utilized fortransmission, the inventive transceiver 700 does not configure any UE tolisten to a “page” during an idle DL sub-frame.

The invention is not limited to the examples of embodiments describedabove and shown in the drawings, but may be freely varied within thescope of the appended claims.

1. A method for use in a wireless communications system wherein a firstnode controls traffic to and from user terminals in a first cell of thesystem, and wherein the first node transmits downlink traffic in radioframes, each frame comprising a certain number of sub-frames, andwherein said method comprises: performing measurements on one or morepre-defined system indicators in at least said first cell; and varyingthe number of available downlink sub-frames which are used by the firstnode for transmission of downlink traffic, according to a decision madebased on results of said measurements.
 2. The method of claim 1, furthercomprising varying the content of the down link sub-frames which areused, according to said decision.
 3. The method of claim 1, furthercomprising autonomously making said decision at said first node,including deciding for how long the decision is valid.
 4. The method ofclaim 1, further comprising communicating the results to a central nodein the system that makes said decision, and receiving said decision fromthe central node in return, for implementation of the decision by thefirst node.
 5. The method of claim 1, wherein said varying comprisesdeclaring one or more downlink sub-frames as downlink idle sub-frames,meaning that no downlink transmission by the first node will take placein those sub-frames.
 6. The method of claim 5, wherein said varyingcomprises declaring one or more previously idle sub-frames as active,meaning that transmission will take place in those previously idlesub-frames.
 7. The method of claim 1, wherein the one or morepre-defined system indicators include the system load in the first cell.8. The method of claim 7, wherein the one or more pre-defined systemindicators also include the system load in at least one other cell inthe system.
 9. The method of claim 1, wherein the one or morepre-defined system indicators include the interference in the firstcell.
 10. The method of claim 1, further comprising said first nodeabstaining from transmitting any of the following items in downlink idlesub-frames: reference symbols; control signaling mapped to the channelsSCH, PCH, or PBCH; downlink scheduling assignments; and schedulinggrants for the uplink.
 11. The method of claim 10, further comprisingscheduling uplink data transmissions so that no data is scheduled in anuplink frame such that HARQ feedback is expected in a downlink idlesub-frame.
 12. The method of claim 10, further comprising configuringuser terminals so that no user terminal is configured to wake up fromdiscontinuous reception (DRX) during a downlink idle sub-frame.
 13. Themethod of 10, further comprising configuring user terminals so that nouser terminal is configured to listen to a page during a downlink idlesub-frame.
 14. A transceiver for use as a first node in a wirelesscommunications system and configured to control traffic to and from userterminals in a first cell of the system, wherein the transceiver isconfigured to transmit downlink traffic in radio frames, with each radioframe comprising a certain number of sub-frames, said transceivercomprising: a transmitter configured for wireless transmission to theuser terminals, including transmission of said downlink traffic, and areceiver configured for wireless reception of uplink signals from theuser terminals; and a control circuit operatively associated with thetransmitter and receiver, and configured to perform measurements on oneor more pre-defined system indicators in at least said first cell, andto vary the number of available downlink sub-frames which are used bythe transceiver for transmission of downlink traffic, according to adecision made based on results of said measurements.
 15. The transceiverof claim 15, wherein the control circuit is further configured to varythe content of the downlink sub frames which are used, according to saiddecision.
 16. The transceiver of claim 14, wherein the transceiver isconfigured to transmit said results to a central node in the system, andto receive said decision in return from said central node, such that thetransceiver varies the number of available downlink sub-frames accordingto the decision taken by the central node.
 17. The transceiver of claim14, wherein the transceiver is configured to make said decisionautonomously, based on said results.
 18. The transceiver of claim 14,wherein the control circuit is further configured to vary the number ofused downlink sub-frames according to said decision, by declaring one ormore downlink sub-frames as idle, meaning that no transmission will takeplace in those idle sub-frames.
 19. The transceiver of claim 18, whereinthe control circuit is further configured to vary the number of useddownlink sub-frames according to said decision, by declaring one or morepreviously idle sub-frames as active, meaning that transmission willtake place in those previously idle sub-frames.
 20. The transceiver ofclaim 14, wherein the one or more pre-defined system indicators includethe system load in the first cell.
 21. The transceiver of claim 20,wherein the one or more pre-defined system indicators include the systemload in at least one other cell in the system.
 22. The transceiver ofclaim 21, wherein the one or more pre-defined system indicators includethe interference in the first cell.
 23. The transceiver of claim 14,wherein, for sub-frames that are not used for down link transmission,the transceiver is configured to abstain from transmitting any of thefollowing items: reference symbols; control signaling mapped to thechannels SCH, PCH, or PBCH; downlink scheduling assignments; and uplinkscheduling grants.
 24. The transceiver of claim 23, wherein thetransceiver is configured not to schedule data in an uplink frame suchthat HARQ feedback would be expected in an idle downlink sub-frame. 25.The transceiver of claim 23, wherein the transceiver is configured notto configure any user terminal to wake up from discontinuous reception(DRX) during an idle downlink sub-frame.